1. Field of the Invention
The present invention relates to a projecting optical apparatus which is used in an over-head projector or a similar apparatus, and more precisely it relates to a projecting optical apparatus in which no trapezoidal distortion is produced when the projecting position (height) is changed.
2. Description of Related Art
FIGS. 6 through 8 show a known over-head projector which will be referred to as OHP hereinafter. In FIGS. 6-8, a light source 12 and a projecting lens system 13 are arranged side by side above a document supporting plate 11 which lies in a substantially horizontal plane, so that the optical axis X of the projecting lens system 13 is normal to the plane of the document supporting plate 11. Above the projecting lens system 13 an inclined reflection mirror 14 is located and has an adjustable inclination angle. Light emitted from the light source 12 is incident upon a document (paper etc.) O on the document supporting plate 11, so that the light reflected by the document O is converged onto a projection screen 15 which lies in a plane perpendicular to the document supporting plane 11 through the projecting lens system 13 and the reflection mirror 14 to make an image on the projection screen 15.
In the OHP mentioned above, the (height position) of an image on the projection screen 15 can be adjusted by controlling the inclination angle of the reflection mirror 14.
In an arrangement shown in FIG. 6, the reflection mirror 14 is inclined at an inclination angle of 45.degree. with respect to the document supporting plate 11 and the projection screen 15, so that the optical axis X of the light transmitted through the projecting lens system 13 and reflected by the reflection mirror 14 is normal to the projection screen 15.
FIG. 7 shows a different position of the reflection mirror 14, in which the reflection mirror 14 is rotated by an angle of 10.degree. from a position shown in FIG. 6 in the counterclockwise direction, so that the light transmitted through the projecting lens system 13 along the optical axis X thereof and reflected by the reflection mirror 14 makes an angle (angle of elevation) of 20.degree. with respect to a line normal to the plane of the projection screen 15.
In the conventional OHP, no trapezoidal distortion of an image O' (which is represented by a grid-like pattern for simplification in FIG. 8) of the document O, as shown by a dotted line in FIG. 8 is produced in a horizontal projection position shown in FIG. 6. However, at an angle of elevation of 20.degree. shown in FIG. 7, a trapezoidal distortion is produced as shown by a solid line in FIG. 8. The trapezoidal distortion increases as the angle of elevation increases. Furthermore, when an image projected on the projection screen has a trapezoidal distortion, not only is the image wholly distorted into a trapezoidal shape, but the light is focused only at one point (focal point) in the vertical direction in FIG. 8. Namely, the image is out of focus.
In actual use of the OHP, the projection state having an angle of elevation as shown in FIG. 7 often occurs rather than the horizontal projection state as shown in FIG. 6. Thus, an image (picture) on the projection screen is viewed at an angle of elevation. Consequently, a viewer looks at an image which is wholly out-of focus.